Electronic switch with common cathode output



Sept. 27, 1949. R. D. GOODRICH, JR,, ET AL 2,482,759

ELECTRONIC SWITCH WITH COMMON CATHODE OUTPUT Filed April 16, 1945.

IN VEN TORS RALPH D. GOODR/CH JR. THOMAS E C. MUCH/MORE Patented Sept. 27, 1949 ELECTRONIC SWITCH WITH COM1VION CATHODE OUTPUT Ralph D. Goodrich, Jr., Laramie, Wyo.,'and Thomas :F. C. Muchmore, .Geneva, vN. Y.

Application April 16, 1945, Serial No. 588,504

2 Claims. (01.250-27) (Granted under the act of "March 3, 1883, as amended April 30, 1928; 3'20 0. G. 757) .The invention described herein ma be manufac'tured and used by or for the Government for governmental purposes, without the payment to us vof any royalty thereon.

The present invention relates to an electric 7 switching device, andit relates more particularly .to such a device wherein switching is accomplished electronically by the employment of vacuum tubes in cathode follower type circuits.

In the electronic art it is often desirable to view two voltage waveforms simultaneously on .the screen of an oscilloscope. In order to do this an electronic switch may be used. In general, the two waveforms to be observed are applied as dual inputs to the switch, and a single output therefrom may be applied to the deflecting plates of an oscilloscope. The output from the electronic switch alternately is proportional to first one, and then the other of the two inputs, and corresponding waveforms appear alternately on the viewing screen of the oscilloscope.

.By synchronizing the oscilloscope sweep circuit .to a common sub-multiple of the frequencies of .the input voltages, and by displacing the alter- .nate output waveforms, the latter appear as two distinct and continuous pictures on the viewing screen. switch may, if desired, be synchronized to the sweep circuits of the oscilloscope.

In prior electronic switches, the input is gen- .erally inverted during the switching action and a resulting finverted waveform appears on the oscilloscope screen. In the study of complex waveforms, this inversion is an undesirable .feature as .it adds to the difficulty of the evaluation .andcomparison of the waveform characteristics.

.In addition, the usual electronic switch has a' high output impedance which'means that the switch and the oscilloscope or other testing device must be in relatively close juxtaposition. Another disadvantag of prior electronic switches is that the vacuum tubes employed in the switching cir- .ouit are capacity coupled, which gives rise to distortion for low switching rates.

It .is therefore an object of the present inven- .tion to provide a novel electronic switch which .overcomes the above disadvantages. In particular, thepresent electronic switch has a reasonably .low output impedance, and the response characteristic is satisfactory over a reasonably broad band of frequencies.

'In the figure, vacuum tubes l and 2 and their associated anode, grid and cathode circuits comprise .a conventional type multivibrator. One multivibrator of this general type is described on The switching rate of the electronic page 182 of Ultra-High Frequency Techniques by Brainerd et al., published July 1942 by D. Van Nostrand Company, Inc. The output of this multivibrator consists of voltages having subgtantia'lly rectangular waveform, where the voltage developed across anode load resistance 3., associated with vacuum tube 1,, is of opposite polarity of that developed across anode load resistance 4 associated with vacuum tube 2.

Switch vacuum tubes 5 and 5 are employed .in cathode follower type circuits and have a ground- "ed, adjustable common cathode load resistance 1. One .of the dual inputs to be switched'is applied to input terminal 8 and the second of the dual inputs may be applied to input terminal ill. Vacuum tubes 5 and 6 are adapted to conduc'talternately (hereinafter described) so that the sis-- nal developed across said. common cathode load resistance 7 is alternately proportional to the inputs applied to terminals 8 and H. The switching action whereby vacuum tubes 5 and 6 conduct alternately can be described as follows.

Assume that buffer vacuum tube ['2 is conducting and that vacuum tube I of the multivibrator starts to conduct. As the latter starts to conduct there 'is a sudden drop in potential across its anode load resistance 3. As the lower end of anode load resistance 3 is connected to control grid 13 of vacuum tube I2 through grid resistance "I4, vacuum tube I2 is driven to cut-off.

Anode 15 of vacuum tube 52 is connected through anode load resistance it (which also serves as the anode load resistance for vacuum tube5) to a suitable source of positive potential ll. As there is only a small amount of distributed capacitance in the anode circuit of vacuum tube [2, the potential of anode it quickly rises to .PProach the voltage of positive potential I 7. Attention is directed to the fact that while vacuum 'tube l2 conducts, the voltage drop across anode load resistance It is such that the potential of anode [8 of vacuum tube 5 is below the potential of cathode2ll of said vacuum tube. Under these circumstances vacuum tube 5 is non-conducting.

This "is due to the fact that when'vacuum tube 1 Anode 25 of vacuum tube 24 is connected through anode load resistance 26 (which also is the anode load resistance for vacuum tube 6) to a source of positive potential ll. When vacuum tube 2% starts to conduct, its anode current rises almost instantly to its final value, and the voltage drop across anode load resistance 26 is suflicient to drive anode 25 of vacuum tube 24'- below the potential of cathode 27 of vacuum tube 6. As anode 25 of vacuum tube 24 is connected directly to anode 28 ofvacuum tube 6, it is apparent that anode 28 is below the potential of cathode 27 and vacuum tube a becomes non-conducting.

In order to prevent distortion of the switching wave forms, when the switching rates are low, direct coupling is employed. The cathode potentials of vacuum tube l2 and vacuum tube 24, and the cathode potentials of vacuum tube 1 and vacuum tube 2 must therefore be chosen so as to allow the proper switching action to take place. Specifically the cathodes of vacuum tubes l2 and 24, respectively, must be at a low enough potential so that the anodes of said tubes, during conduction, will be well below the cathode potentials of vacuum tubes 5 and E. In addition, the cathode potentials of the multivibrator tubes 1 and 2 must be low enough so that the respec tive anodes of said multivibrator tubes drive control grids l3 and 23 of vacuum tubes l2 and 25, respectively, well below cut-01f.

In order to achieve the desired results, cathodes SI and 32 of vacuum tubes 1 and 2, respectively, are connected to a source of negative potential 33. cathodes 34. and 35 of vacuum tubes l2 and 2 3, respectively, are connected through cathode load resistance 38 to said negative potential source 33. As vacuum tubes l2 and 24 conduct alternately, the current flowing through cathode resistance 36 is substantially constant and hence does not have to be by-passed by a capacitance.

In View of the foregoing it is evident that the cooperating action between the multivibrator circuit including vacuum tubes 5 and 2, and the associated vacuum tubes 12 and 2 3, provides substantially rectangular waveform switching voltages at the anodes of cathode follower type vacuum tubes 5 and E5. The switching voltages cause these latter tubes to conduct alternately. Thus, by applying each of the dual inputs (that are to be switched) to corresponding control grid circuits of Vacuum tubes 5 and 8 respectively, an output alternately proportional to each of the dual inputs is developed across cathode load resistance l.

The input to switch tube 5 may be applied to input terminal 8 and the input to switch tube 6 may be applied to terminal ll. Connected across input terminal 8 is potentiometer 31 which acts as a gain control for the input to terminal 8. The signal applied to input terminal 8 is then applied through said gain control potentiometer 3'! through capacitance 38 and isolating resistance 39 to control grid 40 of vacuum tube 5. In a similar manner, the second voltage 4 input is applied to input terminal H thence through gain control potentiometer 42, capacitance 43 and isolating resistance 34 to control grid 45 of vacuum tube 8. Isolating resistors 39 and it serve to prevent the previously described rectangular waveform switching voltages, applied to the anodes of vacuum tubes 5 and 6, from appearing in the input circuits.

When vacuum tube 5 conducts, a voltage proportional to the input applied to terminal 8 is developed across cathode resistance 1 and appears at output terminal it. During this time vacuum tube 6 is non-conducting. During the time vacuum tube 5 is non-conducting, however, vacuum tube 6 is conducting and an output voltage is developed across cathode resistance 1 proportional to the voltage applied to input terminal H.

It is desirable that the alternate outputs appearing across output terminal 36 be superimposed or separated at will on the screen of the associated cathode ray oscilloscope. In general this may be accomplished by having the average anode current from vacuum tube 5 during conduction exceed the average anode current (when conducting) of vacuum tube 6. It is obvious that, if desired, these conditions may be reversed and the (conducting) average anode current of vacuum tube 6 may exceed the (conducting) average anode current of vacuum tube 5. In either case, if no input signals are applied to input terminals 53 and ii a voltage having a substantially -ectangular waveform appears at the output terminal 36. Modulation of the anode currents of cathode followers 5 and 6 by the voltages applied to terminals 8 and H, respectively, result in alternate output voltages substantially proportional to the input signals applied to input terminals 8 and H.

In the absence of any input signals the maximum and minimum voltages appearing across output terminals 66 is dependent upon therespective anode currents flowing through vacuum tubes and 6. By varying the ratio of the anode current of vacumn tube 5, with respect to the anode current of vacuum tube 6, the difference between maximum and minimum voltages across output terminal 38 may be likewise varied.

She control of these anode currents may be obtained by regulating the quiescent potentials of control grids do and iii of vacuum tubes 5 and 5, respectively. In order to more easily position the waveform output on the screen of the oscilloscope, it is desirable to have the anode current of vacuum tube 5 decrease as much as the anode current of vacuum tube 5 increases (and conversely). The corresponding grid potentials should, therefore, likewise increase and decrease by equal amounts. This, of course, assumes that vacuum tubes 5 and operate on the linear portion of their mutual characteristic curves.

One method of varying the bias on control grids it and st of vacuum tubes 5 and respectively, is by means of a voltage divider circuit comprising resistance as, potentiometer 5i and resistances 5s and es. By adjusting the movable arm of potentiometer 5i so that it is in the center of its resistance range, the bias on above-mentioned grids ii} and d5 may be made equal.

This is apparent in that the overall voltage divider circuit described above includes two component voltage divider circuits. The first component divider circuit consists of resistance 52 plus one-half of the resistance of potentiometer 5!, connected serially to resistance 50. The secnd component divider circuit consists of resistance 53 plus the other half of the resistance of potentiometer which is also connected serially to resistance 50. By making resistance 52 equal resistance 53, and connecting them in parallel through the medium of potentiometer 5| it is evident that the total resistance in each component voltage divider circuit may be made equal. Thus, when the movable arm of potentiometer 5| is in the center position of its resistance range, equal bias voltages appear on control grids 5 and 6.

With reference to the figure, if the movable arm of potentiometer 5| is moved to the right, there will be less resistance in the upper portion of the first component voltage divider circuit and the potential of control grid 40 of vacuum tube 5 will rise. In this position there will be a greater amount of resistance in the upper portion of the second component voltage divider circuit and the potential of control grid 45 of vacuum tube 6 will decrease. The position of the movable arm of potentiometer 5| therefore determines the ratio of the anode current of vacuum tube 5 to the anode current of vacuum tube 6. As the effect of the difference in quiescent anode currents determines the displacement of the waveforms on the screen of the associated oscilloscope this control may be referred to as a balance control.

In order to prevent undesired synchronization of the switching rate with the frequency of either of the input voltages, the frequency of the switching is made variable by controlling the multivibrator frequency. One frequency control utilizing a single potentiometer may consist of potentiometer 54, one end of which is connected to positive potential source I1, the other end of which is connected to negative potential source 33. The potential developed at the adjustable contact 55 of potentiometer 54 determines the multivibrator frequency.

Although separate vacuum tubes have been shown, it is to be understood that, if desired, multi-purpose tubes, such as duo-triodes, may be employed. It is to be further understood that other types of vacuum tubes may be used. For example, if vacuum tubes '5 and6 are pentodes having a large mutual conductance (gm) the output impedance of the circuit may be made lower.

It is to be further understood that the term cathode follower switch tube as used in the specification and claims refers to a vacuum tube used in a cathode follower switching circuit.

While there has been here described one embodiment of the present invention employing two cathode follower type switching tubes (5 and 6) the invention herein disclosed is capable of many modifications. For example, a switch may have multiple channels including four or more cathode followers each having a separate grid circuit input and balance control but all having a common cathode load impedance.

It will be further manifest to those skilled in the art that other changes and modifications therein may be made without departing from the invention.

We claim:

1. A circuit adapted to alternately switch two input voltages comprising a first cathode follower switch tube and a second cathode follower switch tube, each having at least a cathode, an anode and a grid, a common cathode circuit for said switch tubes including a load impedance, bias means for said switch tubes, means for applying one of said voltages to the grid of said first switch tube, and

means for applying the second of said voltages to the grid of said second switch tube, means for simultaneously producing two substantially, rectangular waveform switching voltages of opposite polarity, a first buffer vacuum tube, a second buffer vacuum tube, each having at least a cathode, an anode and a grid, the anode of said first buffer vacuum tube being connected to the anode of said first switch tube, the anode of said second buffer vacuum tube being connected to the anode of said second switch tube, means for applying one of said switching voltages to the grid of said first buffer vacuum tube, means for applying the other switching voltage to said grid of said second buffer vacuum tube, the output of said circuit being developed across said load impedance, said output being alternately proportional to each of said voltage inputs.

2. A circuit adapted to switch alternately two input voltages comprising a first cathode follower switch tube and a second cathode follower switch tube, each tube having at least a cathode, an anode, and a grid, a common cathode circuit for said switch tubes including a load impedance, a bias control for the grids of said switch tubes including a voltage source having a positive and a negative terminal, a potentiometer having a movable contact, means connecting one side of said potentiometer to the grid of said first switch tube, means connecting the other side of said potentiometer to the grid of said second switch tube, a resistance connected between one of said terminals and the movable contact of said potentiometer, and two resistances, one side of each being connected to the respective side of said potentiometer, the second side of each resistance being connected to the second of said terminals, means for simultaneously producing two substantially rectangular wave form switching voltages of opposite polarity, a first buffer vacuum tube, a second buffer vacuum tube each having at least a cathode, an anode and a grid, the anode of the first buffer vacuum tube being connected to the anode of said first switch tube, the anode of said 7 second buffer vacuum tube being connected to the anode of said second switch tube, means for applying one of said switching voltages to the grid of said first buffer vacuum tube, means to apply the other switching voltage to the grid of said second buffer vacuum tube, the output of said circuit being developed across said load impedance, said output being alternately proportional to each of said voltage inputs.

RALPH D. GOODRICH, JR.

THOMAS F. C. MUCHMORE REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Review of Scientific Instruments, vol. 12, April, 1941. Combination Vacuum Tube Switch, by Hughes et al., pp. 183-487. (Copy in Scientific Library.) 

